**3. Types of digesters and applications**

The conventional anaerobic digesters operate as semi continuous, continuous or closed. The operations in semi continuous or continuous are preferable because the maximum growth rate can be obtained by controlling the effluent rate. In the closed system, a balance cannot be obtained while the concentrations of the components in the digester change with time (Karakashev & al., 2005).

The choice of the type of digester used is related to treated waste characteristics. Solid waste and sludge are mainly treated in digester with continuous flow (CSTRs), whereas soluble organic waste is treated by a use of biofilm systems such as the anaerobic filters and fluidized bed digesters with ascending flow (UASB) Smith & al., 2005).

In the systems of biofilm the biomass is maintained in the aggregates of the biofilm/granule where the solid retention time (SRT) is much higher than the hydraulic retention time

Production of Biogas from Sludge Waste and Organic Fraction of Municipal Solid Waste 157

with others like sulphur, the phosphorus, the potassium, the calcium, the magnesium and the iron which are required (McMahon & al., 2001). The majority of the nutriments can be

Anaerobic digestion can be applied in a wide range of temperature, into psychrophilous (< 20 °C) (Vavilin & Angelidaki, 2005), in mesophile (25-40 °C), thermophile (45-60 °C) (Angelidaki & al., 2005), and even in extra thermophile conditions (> 60 °C) (Liu, 2003). The temperature has a direct effect on the physicochemical properties of all the components in the digester and affects also the thermodynamics and the kinetics of the biological processes.

1. Increase the solubility of the organic compounds which makes them more accessible to

2. Increase the chemical and Biological reaction rates and hence accelerates the conversion process, therefore the reactor can be smaller and can operate with a shorter hydraulic

3. Improve several physicochemical properties like diffusivity of the soluble substrate, the increase in the rate of transfer of liquid towards gas due to the low solubility of the gas, reduction in the liquid viscosity which makes decreased the energy of agitation

5. Moreover, the reactions of oxidations of organic acid become more energetic at high temperature, which is advantageous for the degradation of fatty acid to long chain fatty acid, and other intermediaries (Chynoweth & al., 1994)**.** Nevertheless, the high temperature can have a certain negative effect. The increase in the temperature decreases the pKa of ammonia, thus increasing the free fraction of ammonia (NH 3) which is an inhibitor of the micro-organisms. Moreover, the rise in the temperature increases the pKa of VFA, which increases its not dissociated fraction, particularly with low pH (4-5), as in the acidogenic reactor (Chynoweth & al., 1994). This makes the thermophilic process more sensitive to inhibition. However, because of the multiplicity of advantages at high temperatures, the thermophilic operation is popular in the anaerobic applications where the ammonia inhibition is not the first

The level of pH has an effect on the enzymatic activity in the micro-organisms, since each enzyme is in activity only in one specific range of pH, and it has its maximum activity with its optimal pH (Ahring, 1994). A stable pH indicates system equilibrium and digester stability. A falling pH decrease can point toward acid accumulation and digester instability. Gas production is the only parameter that shows digester instability faster than pH. The range of acceptable pH for the bacteria participating in digestion is from 5.5 to 8.5, though the closer to neutral, the greater the chance that the methanogenic bacteria will function

necessary and also improves separation liquid-solid separation of the biomass. 4. Particularly increase the death rate of the pathogenic bacteria, which decreases

necessary time for the reduction of pathogenic bacteria (Hansson, 2002).

inhibiting if they are present at high concentrations.

The temperature determines if a specific reaction is favorable. The increase in the temperature has the following advantages:

**4.2 Temperature** 

the micro-organisms.

retention time (HRT)

consideration.

**4.3 pH and buffer value** 

(HRT). The advantage is that the digester can operate with a high flow and can tolerate higher concentrations of toxic species than in (CSTR) systems. The biofilm system operates normally in a continuous mode with an (HRT) lower than 5 days. The systems can operate in a wide range of temperature and psychrophils conditions (3°C) up to the extrathermophiles conditions (80°C). For the anaerobic treatment of soluble organic waste the systems of UASB at high rate are used.

In CSTR systems, the biomass is suspended in the main liquid and will be removed as well as the effluent so that the solid retention time (SRT) is equal to the hydraulic retention time (HRT). This makes it necessary to operate at a high hydraulic retention time (HRT) , between 10 and 20 days, to avoid the scrubbing of the methanogens which have a long time of growth.
